Ever since the demonstration of semiconductor laser diode [1], these light sources have been an expanding field reaching multibillion USD markets. As semiconductor lasers reach higher powers, become more efficient and meet even stricter spectral requirements, new markets are ready to be influenced. At the moment semiconductor lasers are widely used in optical storage, industrial applications and communications. Other notable fields are medicine, sensors, military applications and display systems.
Edge-emitting laser diodes are fabricated using epitaxial semiconductor layers grown on top of a semiconductor substrate wafer. This means that the epitaxially grown semicon-ductor layers, also called epilayers, follow the crystal pattern of the previous layer. Meth-ods of controllably and accurately producing these include molecular beam epitaxy (MBE) and metalorganic chemical vapour phase epitaxy (MOCVD). These laser wafers then undergo fabrication steps including lithography, metallization and cleaving until they are laser bars. The laser devices are often coated as bars and then cleaved into chips.
Laser bars can be seen in Fig. 1.
Laser bars in a gel pack. Bars are usually 10 mm wide, 0.5–4 mm long and contains over 20 individual chips. Image is courtesy of Modulight Inc.
Constant trend in semiconductor lasers is ever decreasing size. Thus one challenge in the development of these lasers is the increasing optical power density. Too high optical power generally leads to sudden device failures at the output. This catastrophic optical damage (COD) breaks the laser. The threshold for COD lowers as the device ages and is exposed to heating, ultimately leading to permanent failure of the laser chip. Lasers op-erated at higher powers are commonly more prone to failures due to high optical power density and high operating temperatures, shortening the laser’s lifetime.
COD is ultimately caused by absorption in the semiconductor structure of the laser or at its resonator. An ideal mirror of a semiconductor laser does not absorb at all. However, semiconductors oxidize quickly and these oxides can generate unwanted surface states near the laser mirrors. At high operating powers these oxides or other impurities absorb some of the light emitted by the laser as well as cause non-radiative recombination, re-sulting in local heating and eventually the failure of the laser device due to COD.
Semiconductor laser facets, which form the optical resonator of the laser, require specific reflectivities for the laser to perform optimally. Because of this laser facets are almost always coated to achieve desired reflectivities. One technique these coatings can be de-posited with is called ion beam sputtering (IBS). It involves an ion source where ions are generated and then accelerated towards a material target. These accelerated ions then knock atoms off of the material target that form a layer on the devices to be coated. This technique provides a controllable method of depositing thin films.
An important benefit of ion beam sputtering, introduced in chapter 4, is that the film can altered by a secondary ion source. Effects on the use of secondary assist source was sys-tematically studied as early as 1963 by Donald Mattox and his coworkers [2]. This sec-ondary ion source can be used to affect properties of the film during deposition or to clean the semiconductor laser facet prior to coating. When the cleaning is done in a controllable manner using ions of the right energy, oxides and other surface impurities can be removed without harming the semiconductor structure underneath increasing the laser’s threshold for COD.
The process of cleaning and coating semiconductor lasers has to happen without interme-diate removal of the devices from vacuum. This is because exposure to ambient air and the oxygen in it causes the surface to reoxidize. Reoxidation happens to some degree during the coating process as well as oxygen is present as sputtering atoms from the ion source or as a background gas. This is because dielectric mirrors are often compounds of oxygen. This problem can be alleviated through an introduction of a passivation layer.
This layer is a thin and inert layer whose purpose is to prevent further reaction of the semiconductor material, thus passivating it.
Effectiveness of cleaning and passivation treatments prior to coating can be evaluated by measuring the COD levels of given devices. It is important to note that COD levels have
a certain amount of randomness in them, requiring a statistical approach. This is because even the slightest contamination in critical location or local defects in the semiconductor structure can cause the COD to happen at different optical power on seemingly identical chips. The laser chips are visually inspected to guarantee that the tested chips are of ade-quate quality.
The aim of this thesis is to find out suitable parameters for an IBS coater assist source to determine the effect of ion source coil power and positive voltage to etching speed and to have an initial parametrization to use the assist source. Different cleaning and passivation treatments can then be tested prior to coating for laser diodes. Then the laser devices of sufficient quality from each coating are tested under pulsed operation to determine the effect of cleaning and passivation treatments to the COD threshold.
This thesis consists of six chapters. In chapter 2 the basics of semiconductors, lasing and related physical phenomena are explained. Also important semiconductor devices are in-troduced. Optical thin films such as anti-reflective and high reflective coatings and their properties are addressed in chapter 3. Chapter 4 focuses on thin film deposition method IBS and possibilities it can add to the coating procedure such as different cleaning and passivation treatments. In this chapter is also explained COD as a phenomenon and sem-iconductor laser diode life time. Lastly, chapter 5 is reserved for results and analysis, and chapter 6 is for conclusions.